1. Field of the Invention
The present invention relates in general to photosensitive polymer films which are used for recording high efficiency holograms. More particularly, the present invention relates to multiple layers of holographic exposures which combine to produce wide bandwidth and superimposed holograms.
2. Description of Related Art
Reflective display holograms have found utility in a wide variety of applications. In addition to their ornamental utility in pendants and jewelry, these holograms are also useful in optical display devices such as head-up displays and windshields which are designed to project images to a viewer. Reflective display holograms operate in the reflective mode and can be illuminated for visualization with white light without significant loss of resolution.
Reflective display holograms traditionally have been prepared by recording the hologram on a photosensitive dichromated gelatin recording medium and subsequently processing the recording to form the hologram. The dichromated gel recording systems consist of a film of the photosensitive dichromated gel on a glass or an optical quality substrate. During the recording process an object wave is reflected from a mirror located a short distance beyond the film of dichromated gel. The reflected object wave and a reference wave form interference fringes which are recorded on the film of dichromated gel.
Dichromated gel recording films are particularly useful for applications in which the desired bandwidth of the hologram is greater than about 30 nm. For example, the dichromated gel recording films can be recorded with a narrow bandwidth laser source. Then, subsequent to recording the laser interference pattern on the dichromated gel film, the film processing can be controlled in a manner which results in a wide bandwidth hologram. The dichromated gel recording films are also conveniently utilized in forming holograms having more than one superimposed image.
A persistent problem presented by dichromated gel holograms is their inherent susceptibility to attack by moisture. Due to the water soluble nature of the gels, great care must be taken during processing and subsequent use to insure that moisture is not present.
Recently, photopolymer recording mediums have been developed which overcome the moisture related problems associated with dichromated gels. These polymeric recording mediums are available from DuPont of Wilmington, Delaware and are generally referred to as holographic recording film (HRF). The exact composition of the photopolymer recording mediums is maintained as proprietary information by the film manufacturers. However, the polymeric recording films are known generally to include polymeric material, photoreactive monomers, initiators, plasticizers and other additives. Processing holographic recording films generally includes high intensity ultra violet radiation curing followed by heating the films in a bake step to finally develop the hologram.
One type of HRF is marketed by Dupont Co. Imaging Systems, Wilmington, Delaware under the trade name OMNIDEX. OMNIDEX photopolymer recording medium is available in a liquid form which can be applied to various surfaces to form an HRF. OMNIDEX photopolymer recording medium is also available as a prefabricated film which is sandwiched between two layers of MYLAR brand polyester. One of these layers is generally referred to as the base layer while the second layer is a thinner cover layer. The cover layer functions as a protective barrier and is particularly useful for preventing the layer of photopolymer from coming into contact with index matching oils during the recording process. The index matching oils interfere with the physical stability of the photopolymer and can decrease the usefulness of the photopolymer recording film.
Photopolymer holographic recording films are inherently resistant to attack by moisture. Accordingly, they are well suited for use in making high efficiency holograms which may intentionally or inadvertently come into contact with moisture. One disadvantage associated with these photopolymer based holographic recording films is that they provide restricted narrow bandwidth holograms. Inherently, the bandwidths of photopolymeric holograms are on the order of 25 nm and do not go beyond 30 nm.
As mentioned earlier there are applications in which a controlled bandwidth of several hundred nanometers is desired. There are additionally applications in which more than one and frequently several holographic superimposed images are desired. For example, windshields having one hologram for projecting images to a viewer and a second hologram which functions as a reverse light blocking hologram and prevents light from entering the car are being contemplated. Each of these holograms is independently recorded and frequently independently processed yet they are incorporated into a single device.
One approach to solving the problem related to obtaining a wide bandwidth hologram when using photopolymer holographic recording films is to bond together separately exposed and processed holograms, each having a different wavelength hologram. The combination of all the separate holograms in a stack results in an apparently single hologram diffracting at a wide range of wavelengths. The combination of all the separate holograms can also display a number of apparently superimposed images or recordings. This approach to overcoming the problems associated with the inherently narrow holographic bandwidths of photopolymer recording film has the disadvantage of having a potentially large number of bulky and potentially visually interfering adhesive layers.
Accordingly, it would be desirable to provide a method for preparing multiple independent layers of photopolymer based holograms wherein the combination of all the holograms diffracts at a wide range of wavelengths and wherein the multiple independent layers are not adhered together with bulk adding adhesive layers. It would also be desirable to provide a method for preparing a holographic stack of layers of photopolymer based holograms wherein the combination of holograms presents several superimposed images. It would also be desirable to provide a method for preparing a holographic stack in which the layers of photopolymer have a protective barrier during the recording process.